Evaluation of Wear Resistance of AISI L6 and 5140 Steels after Surface Hardening with Boron and Copper

Abstract

(1) Background: Boriding is one of the most common methods of thermal-chemical treatment due to its excellent hardness and wear resistance of the produced diffusion layers. However, it has limited application compared to carburizing and nitriding because of fragility and chipping. Introducing another alloying element into the boron media helps avoid those drawbacks and improve other surface properties of the layer. The purpose of this work is to improve the surface mechanical properties of L6 and 5140 low alloy steels by two-component surface hardening with boron and copper. (2) Methods: The treatment was performed by means of a powder-pack method using boron, copper, and aluminum powders in the following proportions: 60% B₄C + 20% Al₂O₃ + 16% CuO + 4% NaF. The time–temperature parameters of the treatment were four hours exposure at 950 °C. Microstructure, elemental, and phase composition were investigated as well as microhardness and wear resistance of the obtained layers. (3) Results: Layers of up to 180–200 μm thick are formed on both steels as a result of treatment. Needle-like structures similar to pure boriding was obtained. The maximum microhardness was 2000 HV on L6 steel and 1800 HV on 5140 steel. These values correspond to iron borides and were confirmed by XRD analysis revealing FeB, Fe₂B, and Cr₅B₃. The wear resistance of both steels was about ten times higher after the treatment compared to non-treated samples. (4) Conclusions: Surface hardening with boron and copper significantly improves the mechanical properties of both alloy steels. The results obtained are beneficial for different tribo-pair systems or three-body wear with abrasion and minimum impact loads.

1. Introduction

Surface hardening is one of the main ways to increase the wear resistance of parts and frictional units. In industry, there are more than a hundred methods of surface hardening, but not all have found industrial application. The most well-known methods include carburizing, nitriding, surfacing, and boriding [1].

Some samples are difficult to subject to surface hardening due to dimensions and complex geometry. They were initially designed with no consideration given for the reconstruction or repair of the units. Not all parts follow the “industrial recycling code”, which requires that worn surfaces of machine parts be rebuilt for reuse, and that rebuilding is cheap and convenient, and it must be provided for at the design stage [2].

A wide range of materials, including ferrous and non-ferrous metals are subjected to the process of thermal-chemical treatment (TCT). One of the well-known TCT methods is boriding, aiming to improve the physical and mechanical properties of steels and alloys by applying a boride layer [3,4,5,6]. The obtained boride layers have sufficient adhesion, high resistance to abrasion wear, and oxidation. However, it is of limited use because of its brittleness and chipping. Introducing another alloying element into boron media helps avoid those drawbacks and improve other surface properties of the layer. Multicomponent TCT methods such as boroaluminizing, borochromizing, and boronickelizing are reported in previous works [7,8,9,10].

Borocoppering or boron-coppering (according to Krukovich [11]) is also considered as one of the promising methods of TCT. A review of works on this topic showed that most studies have been conducted in the post-Soviet countries, such as Belarus and Russia [12,13,14,15]. The simultaneous diffusion of boron and copper leads to an increase in the thickness of the boride layer and a decrease in its brittleness. According to [12,13], copper oxide introduced into the saturating powder mixture halves the processing time from four to two hours. The effect is achieved by a reduction of copper in an ammonia atmosphere in a fluidized bed. The reaction then produces a layer of copper, which promotes the subsequent growth of the boride layer.

In terms of terminology, surface hardening or TCT with boron and copper is used in this paper instead of borocoppering because coppering is a process of copper plating by the electro-chemical process [15]. The authors’ previous research has focused on powder mixture optimization. The ratio of copper oxide in the saturation mixture in powder boriding is essential for the growth of a sample size, surface roughness, and layer thickness [12,16,17]. It has been found that the copper oxide content of more than 15 wt % results in a thicker boride layer with high roughness (Ra) up to 0.727 µm (compared to initial Ra of 0.01 µm), which increases the size of the sample by 85–135 µm on each side. Moderate results with adequate correspondence between layer thickness and surface properties have been obtained at 12–15 wt % of copper oxide in powder mixture.

The purpose of this study is to implement the next stage of research and improve wear resistance of low alloy steels and reduce the brittleness of the boride layer by adding a copper compound to the boriding media.

2. Materials and Methods

The objects of the study are 5 KhNM and 40 Kh low alloy steels by Russian marking, analogues of AISI L6 and 5140 steels, respectively. The AISI steel grade is further used in the paper. The chemical compositions of the steels are given in

Table 1. The chemical composition of L6 and 5140 steels, wt %.

Steel Grade	C	Mn	Si	Cr	Ni	V	Mo	Fe
L6	0.65–0.75	0.25–0.8	0.25	0.6–1.2	1.25–2	0.2–0.3 1	up to 0.5	94.2–97.0
5140	0.38–0.43	0.7–0.9	0.15–0.35	0.7–0.9	-	-	-	97.42–98.07

¹ Optional.

. It should be noted that in Russian steel grades of 5140 and L6, the Cr content is 0.8–1.1 and 0.5–0.8%, respectively.

AISI L6 tool steel is a multipurpose, oil-hardening steel characterized by advanced toughness. The relatively low carbon and high nickel content makes it possible to produce an alloy with greater impact toughness than other common oil-hardening grades, and should be used where some wear resistance can be sacrificed for increased toughness [18].

AISI 5140 steel is a structural alloy steel for general application. It is widely used in low- and moderate-load parts for vehicles, engines, and machinery where a hard, wear-resistant surface is required [19].

The size of the steel samples was obtained as 15 mm × 12 mm × 5 mm. Surface saturation of the samples was carried out in powder saturating mixtures (60% B₄C + 20% Al₂O₃ + 16% CuO + 4% NaF) in cylindrical crucibles; fusible glass was used as a sealing shutter. After packing the samples, the crucibles were placed in a muffle furnace for 4 h, and the holding temperature was 950 °C. After the exposure, the crucibles were cooled in the air, unpacked, and the samples were removed, washed, and cleaned of saturation mixture residue. After that, cross sections were prepared for metallography studies using abrasive paper with different grain sizes. An ALTAMI MET 2S optical microscope was used to study the microstructure and the thickness of the diffusion layer. The microhardness was measured by a PMT-3 microhardness tester with a load on the diamond pyramid of 0.05 kg. Elemental microanalysis was performed with a JEOL JCM-6000 scanning electron microscope (SEM) with elemental dispersion analysis. The studies were carried out using a detector of secondary electrons in a high vacuum mode, the accelerating voltage was 15 kV. X-ray phase analysis was executed on a D2 PHASER diffractometer with a LYNXEYE linear detector in copper radiation with a shooting interval of 15–90°. The measurement step was 0.020°, and the processing time for one step was 1.2 s.

Wear resistance was determined on the friction machine (Model 1, Research Center “Scientific Instruments” of the Buryat State University, Ulan-Ude, Russia), which is shown in Figure 1a, according to the block-on-ring scheme simulating the sliding wear behavior (Figure 1b). The sliding speed was 1 m/s, the load was 627 N/m. The material of the counterbody was 1045 hardened steel with a diameter of 50 mm and a hardness of 57 ± 2 HRC. Fine turning was applied as a finishing treatment for the counterbody ensuring surface roughness Ra of 1.6–1.8 µm. A non-contact infrared thermometer AR 872A with a temperature range of −50–+1150 °C was used to determine the temperature of the counterbody and the sample. Weight measurements of the samples were recorded every 10 min on an A&D Weighing Galaxy HR-100A analytical balance (A&D Weighing, San Jose, CA, USA). The test lasted until a dramatic weight loss was detected, which usually indicated the failure of the diffusion layer and wear of the base metal beneath it. Wear test for each sample was repeated three times.

3. Results

3.1. Microstructure, Microhardness and Phase Composition

Diffusion layers up to 200–220 µm thick on L6 steel and up to 180–200 µm thick on 5140 steel were obtained by complex saturation of sample surfaces with boron and copper (Figure 2).

The diffusion layer on the 5140 and L6 steel samples has an overall needle-like structure. Needle coarsening is observed in the upper zone of the layer on both steels where they form a continuous layer up to 100 µm thick. The presence of pores on the surface of the layers is presumably due to the initial disoriented boride crystals formed over the steel surface at the first stage of TCT. In addition, the transformation of Fe into Fe₂B is due to a 16% increase in volume [20,21]. At the same time, they can work as an oil-retaining compartment in different friction pairs in the presence of a lubricant. The microstructure of the 5140 steel base has a ferrite–pearlite structure characteristic of hypoeutectoid steel. The transition zone between the diffusion layer and the metal base is not observed. L6 steel has a microstructure consisting of lamellar pearlite with minor inclusions of ferrite.

The highest microhardness of L6 steel is 2000 HV, which refers to FeB boride [22,23]. It is known that the hardness value reflects the boron content of the boriding. The highest hardness is achieved in comparatively thick diffusion layers with the FeB as the external phase. The drop in microhardness at a depth of 120–150 µm is presumably related to FeB/Fe₂B interfacial tension.

The maximum microhardness (about 1800 HV) of the 5140 steel sample is lower compared to the L6 steel sample. The hardness gradually decreases from the surface to the base metal without significant fluctuations until it reaches a plateau with a hardness of the 200–300 HV (Figure 3).

XRD analysis of L6 steel showed that the diffusion layer consists of three borides: FeB, Fe₂B, and Cr₅B₃ (Figure 4). It is known that copper does not form stable compounds with either boron or iron [24,25]. Copper was found on the surface of the L6 steel. The presence of two borides, Fe₂B and Cr₂B, was revealed in the diffusion layer of 5140 steel (Figure 5). In addition, one carbide Cr₇C₃ was identified. A higher amount of chromium led to the formation of the observed presence of chromium in its pure form. It should be noted that the presence of higher FeB boride is not observed, which is due to the fact that chromium retards diffusion of boron into the base metal [26].

Previous results have showed that 4 h exposure has a greater effect on the layer thickness growth rate than 5 h exposure in borocoppering [27]. Thus, the layer thickness was 60 µm, 160 µm, and 180 µm after 3, 4, and 5 h exposure. This corresponds to findings on pure boriding, where the authors described the growth of the diffusion layer thickness as a parabolic dependence on the holding time [28]. As a result of surface hardening AISI L6 steel for 0.5–3 h at 900, 950, and 1000 °C, boride layers from up to 180 µm are obtained [29]. Boriding of AISI 5140 steel in a new powder mixture Baybora-1 at 950 °C for four hours leads to 90 µm thick diffusion layer, which is twice lower than after TCT with boron and copper [30].

3.2. Wear Test

The wear test indicated that the samples after the TCT outperformed samples without treatment by several times (Figure 6). Both steels had a similar mass loss rate in the bare state which is characterized by a dramatic loss of mass from the very beginning of the test. After the TCT, both steels demonstrated sustained wear resistance up to 90 min testing. Between 90 and 150 min of testing, mass loss slightly increased from 0.008 to 0.015 g. After 150 min of testing, a significant mass loss was found for both steels, indicating that the diffusion layer had failed. Further tests indicated a differentiation slope of the mass loss depending on the steel grade. Due to the higher concentration of alloying elements and better mechanical characterization of L6 steel, its mass loss was lower compared to 5140 steel. The slope angle in the last stage of testing characterizes a complete wear of the diffusion layer and achieves the base metal.

The wear process was accompanied by removing micro asperities and separation of small particles, which, in turn, acquired the simulated nature of abrasive wear. Finally, the particles were removed from the interaction zone, which led to an increase in the contact area. As a result, the heat generated by friction was provoked [31]. A direct correlation between the wear test duration and the temperature in contact zone was shown in Figure 7. The maximum temperature values for L6 and 5140 steels were measured as 178 °C, and 185 °C, respectively. It should be noted that the temperature reached a maximum after 120 min of testing for 5140 steel, while for L6 steel, it continued to rise until the end of the test.

It is known that friction-induced heating accelerates tribooxides formation [32]. The EDS analysis of the worn surface on L6 steel showed that all seven spectra across the worn track contain oxygen confirming surface oxidation (Figure 8a,

Table 2. The elemental composition of L6 steel in worn surface on Figure 8a.

No Spectrum	Elemental Composition, wt %
	B	C	O	Al	Cr	Ni	Cu	Mo	Fe
1	-	23.18	8.07	5.86	2.22	1.48	5.68	-	53.51
2	0.7	38.73	3.50	0.08	-	0.02	3.00	-	53.97
3	0.51	79.44	4.47	-	-	-	-	-	15.58
4	-	14.92	17.98	0.28	0.62	0.03	0.53	4.26	61.38
5	0.85	-	21.19	-	-	64.84	13.12	-	-
6	-	99.85	0.05	-	-	-	-	0.10	-
7	-	-	20.86	-	-	45.03	34.11	-	-

). As for the diffusion elements, boron content was up to 0.85%, and copper was up to 34.11%. High concentrations of copper, nickel, chromium, iron, and aluminum can be explained by the creation of oxide film on the worn surface. The latter element was in the treatment powder as a balancing medium. The high carbon content can be explained by contamination during the test. The surface of 5140 steel was also highly oxidized after the wear test (Figure 8b,

Table 3. The elemental composition of 5140 steel in worn surface on Figure 8b.

No Spectrum	Elemental Composition, wt %
	B	C	O	Al	Cr	Ni	Cu	Mn	Fe
1	2.48	47.76	12.86	1.68	0.33	0.29	0.67	-	33.93
2	0.4	15.68	19.62	1.72	0.29	0.10	-	-	62.19
3	0.4	24.84	17.57	1.55	0.24	0.03	0.32	0.46	54.59
4	0.03	22.46	19.00	3.53	0.78	0.06	0.98	-	53.16
5	-	25.34	18.10	3.06	0.35	0.78	0.10	-	52.27
6	-	23.58	18.61	3.25	0.43	0.46	0.81	0.19	52.67
7	-	20.22	19.68	4.10	0.51	1.25	2.22	-	52.02

).

4. Discussion

It is well-known that the formation of oxides and oxide films depends not only on the physical and chemical nature of the oxidized metal, but also on the conditions of its formation, mainly temperature [33]. For example, FeO—iron monoxide, is stable at temperatures above 570 °C and cannot form at lower temperatures [34]. Copper oxide CuO is presumably formed at a temperature of 200 °C and higher [35]. As was mentioned above, Al₂O₃ is from the treatment powder. From the above, an assumption is formed that the formation of these oxides is not caused by an increase in temperature during wear tests, although the maximum temperature during wear tests was 160–180 °C (Figure 7). The source of some oxides is probably the reaction with atmospheric oxygen during TCT in powder mixtures at 950 °C. The two sources of oxygen are poor sealing during the TCT process as a result of insufficient sealing of containers and atmospheric oxygen between particles in the powder mixture. However, it should be noted that friction in the absence of lubrication between the surfaces of the parts may be accompanied by the appearance of microsparks on the projections of the roughness of the metals, in which the temperature of the microspark can reach quite high values, and under conditions of friction in the air atmosphere, contributes to the formation of metal oxides of the tribosystem. According to Luo, four types of tribofilms can form between sliding surfaces, where tribofilms generated from the chemical reaction between the wear products and the environment belong to the fourth type [32]. Mechanism of tribooxide formation probably takes several steps before interacting with atmospheric oxygen. First, debris particles are formed due to surface asperities and protrusions fracture. As the sliding continues, the particles become smaller, contributing to their higher surface to volume ratio. At that point, the adsorbed molecules of oxygen and water moisture break the chemical bonds of the debris particles. In addition, friction-induced heating promotes these chemical reactions in the tribosystem, resulting in complex multi-component oxides [32]. It is known that tribofilms significantly contribute to the wear resistance, reducing the friction coefficient and wear rate of boron and nitride–phosphate coatings [36,37]. Moreover, copper lubricant additive reduces the wear rate and lowers friction coefficient of 100Cr6 bearing steels tribopairs during the friction tests [38]. TCT with boron and copper can be recommended instead of pure boriding due to its superior diffusion layer thickness and lower fragility [27].

5. Conclusions

Boride layers with a thickness of 180–220 μm and the acicular structure were obtained on the surface of 5140 and L6 steels as a result of borocoppering. XRD analysis showed the presence of iron and chromium borides on both steels. However, iron borides FeB were found on the surface of L6 steel, which contributed to its higher microhardness of 2000 HV compared to 1800 HV on 5140 steel. Other hard compounds were Fe₂B, Cr₂B, Cr₇C₃, and Cr₅B₃.

Borocoppering significantly enhanced wear resistance of L6 and 5140 steels. It was found that wear resistance of both the steels was about ten times higher after the treatment compared to non-treated samples. Friction-induced heating up to 185 °C contributed to the debris particle oxidation process. As a result, tribofilms made of complex oxides were formed during the test.

Surface hardening with boron and copper significantly improves the mechanical properties of both the alloy steels. Adding copper into the boriding media resulted in a thicker diffusion layer with better mechanical properties. The obtained results are beneficial for different tribo-pair systems or three-body wear with abrasion and minimum impact loads, e.g., bushes, pipe bends, baffle plates, runner, helical gear drive for oil pumps, pump shafts.